JP H6-101534 A describes an evaporated fuel processing device. The evaporated fuel processing device includes a sensor for specifying a fluid density of air introduced into a canister and a sensor for specifying a fluid density of a purge gas sent from the canister to an engine. The sensor for specifying the fluid density of the purge gas is disposed between the canister and an intake passage leading to the engine. The evaporated fuel processing device uses the fluid density of the air and the fluid density of the purge gas specified respectively by the two sensors while purge is executed from the canister to the engine to calculate a concentration of the purge gas based on a ratio or a difference of these fluid densities.